The present invention is related to chemical etchants and processes for removing coatings from metal parts, particularly to the removal of corrosion resisting coatings from nickel superalloys.
High temperature superalloys, such as the alloys U-700, IN-100, MAR M-200 and the like, designed for use at high temperatures in gas turbine engines, are especially strong and resistant to oxidation and corrosion at high temperatures. However, the design of superalloy compositions always involves trade-offs between improved corrosion resistance and improved strength. For this reason, superalloy components often are surfaced with coatings of materials specially formulated to resist corrosion.
During use, parts become worn or damaged to the point where they must be restored using various processes, such as machining, shaping, and welding. In these processes it is often necessary to subject the part to a high temperature, or expose it to a repeat of the original heat treatment, during which the coating would undesirably interact with the substrate. Because of this, and because the old coating may be uneven and itself deteriorated, it is necessary to remove, or strip, the old coating from the part. Because of the irregular contours of parts such as gas turbine blades, this often cannot be conveniently done by mechanical means. Furthermore, mechanical abrasion has the disadvantage of inevitably removing some of the substrate which parts having critical dimensions cannot withstand. Consequently, chemical stripping methods are preferred. Typically, a part is immersed in a chemical solution which attacks the coating. However, stripping is not easily done because the very nature of the coating is that it is resistant to chemical attack in general. Furthermore, a chemical solution sufficiently strong to attack the coating in an economically feasible time also tends to attack the substrate material, which is particularly disadvantageous if it results in localized attack at the grain boundaries. The substrate is thereby weakened, and restoration of the part to service becomes impossible.
The present invention is particularly addressed to the problem of removing an aluminide coating from nickel base superalloy. Typical composition of such a coating would be that obtained by a pack cementation process using aluminum silicon alloy powder, such as referred to in U.S. Pat. No. 3,544,348 to Boone et al. Basically, the coating on the finished part is nickel aluminide, NiAl. Various chemical solutions have been used heretofore for stripping aluminide coatings from nickel superalloys. In the practice, the component is repetitively immersed in an acid solution, rinsed in water, dried, grit blasted and re-immersed in the acid, etc. Solutions which have been used are, by volume, 20% nitric acid, balance water; 12.5% nitric acid, 5% phosphoric acid, balance water; 15 gm/liter water of proprietary Metex M628 dry acid salts (Mac Dermid Corp., Waterbury, Connecticut); and a mixture of nitric acid, water and proprietary solution ASC-2-N (Alloy Surfaces, Inc., Wilmington, Delaware). With the most favored 20% nitric acid solution, during each immersion vigorous agitation is required to prevent local pitting. This means that any areas of a component, such as recesses or cavities which cannot be easily flushed, are potentially prone to localized pitting which may degrade the mechanical strength of the component. Coating removal is slow, but the total immersion time in the acid solution must not exceed 7 hours, since it has been determined that beyond this time the substrate will be adversely attacked intergranularly.
Thus, there is a need for an improved method for removing aluminide coatings which the present invention fulfills. The invention is related to copending application Ser. No. 192,668, "Selective Chemical Milling of Recast Surfaces," filed Oct. 1, 1980 by the same inventors hereof; described therein is selective chemical milling of recast layers resulting from localized melting of superalloys, such as those based on MAR M-200 alloy. A somewhat lesser degree of relationship will be found with application Ser. No. 192,667, "Chemical Milling of High Tungsten Content Superalloys," filed Oct. 1, 1980 by the common inventors hereof, together with Manty; disclosed are solutions for chemical milling superalloys having high tungsten contents.